Liquid crystal displays (LCDs) continue to improve in cost and performance, becoming a preferred display type for many computer, instrumentation, and entertainment applications. The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light providing surface positioned behind the LCD for directing light outwards, towards the LCD. The challenge of providing a suitable backlight apparatus having brightness that is sufficiently uniform while remaining compact and low cost has been addressed following one of two basic approaches. In the first approach, a light-providing surface is used to provide a highly scattered, essentially Lambertian light distribution, having an essentially constant luminance over a broad range of angles. Following this first approach, with the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution in order to provide a more collimated illumination.
A second approach to providing backlight illumination employs a light guiding plate (LGP) that accepts incident light from a lamp or other light source disposed at the side and guides this light internally using Total Internal Reflection (TIR) so that light is emitted from the LGP over a narrow range of angles. The output light from the LGP is typically at a fairly steep angle with respect to normal, such as 70 degrees or more. With this second approach, a turning film, one type of light redirecting article, is then used to redirect the emitted light output from the LGP toward normal. Directional turning films, broadly termed light-redirecting articles or light-redirecting films, such as that provided with the HSOT (Highly Scattering Optical Transmission) light guide panel available from Clarex, Inc., Baldwin, N.Y., provide an improved solution for providing a uniform backlight of this type, without the need for diffusion films or for dot printing in manufacture. HSOT light guide panels and other types of directional turning films use arrays of prism structures, in various combinations, to redirect light from a light guiding plate toward normal, or toward some other suitable target angle that is typically near normal relative to the two-dimensional surface. As one example, U.S. Pat. No. 6,746,130 (Ohkawa) describes a light control sheet that acts as a turning film for LGP illumination.
Referring to FIG. 1, the overall function of a light guiding plate 10 in a display apparatus 100 is shown. Light from a light source 12 is incident at an input surface 18 and passes into light guiding plate 10, which is typically wedge-shaped as shown. The light propagates within light guiding plate 10 until Total Internal Reflection (TIR) conditions are frustrated and then, possibly reflected from a reflective surface 142, exits light guiding plate at an output surface 16. This light then goes to a turning film 20 and is directed to illuminate a light-gating device 120 such as an LCD or other type of spatial light modulator or other two-dimensional backlit component that modulates the light. For optimized viewing under most conditions, the emitted light should be provided over a range of relatively narrow angles about a normal V. A polarizer 124 is typically disposed in the illumination path in order to provide light-gating device 120 such as a liquid crystal cell with suitably polarized light for modulation. A reflective polarizer 125 is often provided between absorptive polarizer 124 and turning film 20.
Referring to FIG. 2, there is shown a schematic cross-sectional view of a conventional turning film 20a used with light guiding plate 10, showing key angles and geometric relationships. Turning film 20a has a number of prismatic structures facing downward toward light guiding plate 10, each structure having a near surface 24 (being near relative to light source 12, as shown in the embodiment of FIG. 1) and a far surface 26, both sides slanted from a film normal direction V as determined by an apex angle α, and base angles β1 and β2, relative to a horizontal S. Light from light guiding plate 10 is incident over a small range of angles about a central input angle θin. The output angle θout of light delivered to the LC display element at a flat surface 22 of turning film 20a is determined by a number of factors including the central input angle θin, the refractive index n of turning film 20a, and the base angle β1 at which far surface 26 is slanted. Output angle θout for emitted light is preferably normal with respect to turning film 20a, however output angle θout can be considered a target angle, which may be at some inclination with respect to normal for some applications. For most conventional turning films, the target angle is normal. In a typical arrangement, base angles β1 and β2 are about 56 degrees, and apex angle α, 68 degrees. The primary ray 50a having an input angle around θin≈70° is redirected to near normal direction. However, some secondary rays 50c, 50c1 having an input angle around θin<70° may take paths as shown in FIG. 2. Secondary ray 50c1 is redirected toward a relative large angle from the normal direction. Further, secondary ray 50c is totally reflected back by the light exiting surface 92. Consequently, the light utilization of this existing turning film is not satisfactory.
Thus, while there have been solutions proposed for turning films suitable for some types of display apparatus and applications, there remains a need for improved turning films.